Natural cellulose fibers, such as those obtained by the pulping of wood by known paper pulp making processes, may be substantially modified in their physical and chemical properties by graft polymerizing selected olefinic monomers in and on the fibers by known reactions carried out in situ in aqueous suspensions of the cellulose fibers. For example, natural cellulose fibers modified by the presence of alkali metal salts of polyacrylic acid chemically united with the cellulose, may be obtained by an in situ polymerization of acrylonitrile in and on papermaking cellulose pulp fibers, followed by a hydrolysis in strong alkali of the nitrile groups present in the polyacrylonitrile portions of the polymer-modified fibers. The preparation of such polyacrylate-modified cellulose fibers has been described in prior art patents, including U.S. Pat. Nos. 3,194,727, 3,256,372, 3,366,582 and 3,682,856, issued to J. W. Adams and H. W. Hoftiezer, 3,793,299 to Zimmer, and, more recently, 3,838,077, issued to H. W. Hoftiezer and A. H. Tilloson, which latter patent describes an improved method for hydrolysis of the polyacrylonitrile-modified fibrous cellulose material at high solids concentration to achieve a material having superior physical properties for use as a viscosity builder and absorptive agent for aqueous media.
To be useful as a principal absorbent medium to be incorporated in disposable diapers and other single use absorptive pads, the absorptive medium must have the capacity to absorb large amounts of fluids rapidly and to retain the absorbed fluids completely, even though the absorptive pad is subjected to twisting, flexing or pressure forces while being worn or held in contact with an active human body. At the same time, the absorbent medium must retain its structural integrity and not shift position substantially in relation to the other pad components, even though the pad is subjected to normal stresses and pressures while substantially saturated with body fluids or exudates.
Certain polymer-modified natural cellulose fibers are well suited to use as an absorptive medium in diapers and other disposable pads. Of particular value are insoluble, fibrous products prepared by treating an aqueous suspension of natural cellulose fibers, such as those obtained by standard pulping procedures from wood, straw or other lignocellulosic plant material, or cellulose fibers from cotton linters, etc., with an ethylenically unsaturated compound containing at least one functional moiety selected from the group consisting of acrylamide, acyl halide, nitrite, ester, alkenyl halide, carboxylate salt and carboxylic acid in the presence of a polymerization catalyst and subsequently hydrolyzing or neutralizing the resultant polymer-modified cellulose fibers under controlled alkaline conditions to obtain alkali metal polymer-modified cellulose fibers. In the preparation of the polyacrylonitrile-modified cellulose product, for example, the reactant ratios are adjusted to yield a product having a polymer-to-fiber ratio of between about 0.5 to 1 and about 5 to 1. If such a product is then subjected to an alkaline hydrolysis with an alkali metal hydroxide, the hydrolysis conditions and subsequent treatment of the reaction product will profoundly influence the properties relating to its utility as an absorptive medium in disposable pads as described hereinbelow.
For example, if the hydrolysis is carried out at low total solids concentration (up to about 25% total solids, for example), the resultant product, although having a high absorption and retention for aqueous fluids, will lack structural integrity when wet, the individual damp fibers easily moving in relation to each other so that the absorbent material readily shifts position in the pad under stress of any kind. The product is thus unsatisfactory for general use as an absorbent medium in a diaper, for example, since the absorbent material, when moist, will be displaced by an infant's weight and movements from the particular areas where absorptive potential is critically needed. If, on the other hand, the hydrolysis is carried out under particularly drastic conditions of temperature and of reagent concentration, the resultant product, while displaying excellent structural integrity so that it retains its position in a composite diaper construction under all normal circumstances of stress or pressure, is nevertheless of less than optimum suitability because of its relatively low fluid absorptive and retentive capacity. Polyacrylate-modified cellulose fibers prepared in accordance with the alkaline pressure hydrolysis described in U.S. Pat. No. 3,793,299 issued to R. E. Zimmerer for synthesis of a non-swelling cellulose-acrylate copolymer of high ion exchange capacity fall in this category.
Even more recently, Lepoutre in U.S. Pat. No. 4,025,472 and in a paper, "The Production of a Fibrous Superabsorbent: Technical and Economic Aspects," PPRIC of Canada, March, 1976, has proposed methods to provide wood pulp modified by graft-polymerization of polyacrylonitrile which is subsequently hydrolyzed to give improved water absorption properties.
In general, thickeners, adhesives or super absorbents may be produced by hydrolyzing graft copolymers of acrylonitrile and cellulose in wood pulp resulting in at least three different products in terms of chemical and physical properties which may be loosely characterized as (1) difficult-to-dry, discreet fibers types derived from paste hydrolysis; (2) easy-to-dry, hard gel fiber clusters types derived from crumb hydrolysis and (3) hard fiber aggregates types derived from high pressure hydrolysis.
To understand the differences between the various products and procedures, a consideration of the course of the hydrolysis is helpful. For example, alkaline hydrolysis of the nitrile groups on polyacrylonitrile to sodium carboxylate groups proceeds through an amide intermediate: ##STR1## Because of neighboring group effects, hydrolysis is never complete and a combination of carboxyl and amide groups is obtained. The ratio of carboxyl to amide groups in hydrolyzed polyacrylonitrile modified-cellulose products therefore depends on reacting conditions.
In the case of paste hydrolysis where alkali concentrations range from about 1.0 to 15% in water, the polyacrylamide content of cellulose grafts ranges from about 40 to 60%. The products are discrete fibers and although paste hydrolysis procedures are capable of yielding highly absorbent product as indicated by LePoutre, the actual product yield in general has been very low, as much as 50% or more of product in general being lost in washing. Additionally, after washing, the paste of swollen fiber fragments is very difficult to dry so that solvent treatment to agglomerate fibers and remove water is required.
Easy-to-dry, hard gel fiber clusters from crumb hydrolysis after aging in accordance with U.S. Pat. No. 3,838,077, and after grinding, are suitable water-base thickeners but exhibit only poor-to-fair qualities as superabsorbents while hard-gel aggregates as in U.S. Pat. No. 3,793,299 are suitable primarily as ion-exchange resins.